The present invention relates to an ink jet fluid system, and more particularly, to a system and process for the startup and shutdown of the print head of an ink jet printer including a system and process for maintaining independent control of the fluid pressure within individual print heads in a multiple print head device.
Ink jet printers are known in which ink is supplied at a pressure in excess of atmospheric pressure to a manifold communicating with a series of small diameter orifices. As the ink flows through the orifices under pressure, it forms fine filaments of fluid which break up into jets of discrete drops. At the point at which the drops break away from the filaments, they pass through electrically conductive charging rings to which charging potentials are selectively applied to charge selected ones of the drops. An electrostatic deflecting field, extending across the paths of the jet drop streams, deflects the charged drops away from their initial trajecturies in amounts which correspond to their levels of charge. A catcher is positioned for catching those drops traveling along predesignated catching trajectories. Those drops which are not caught are deposited upon a print receiving medium which is transported beneath the printer.
The startup, control, and shutdown of the pressurized ink systems in such ink jet printers without excessive weeping or splattering of ink over the system components or onto the print receiving medium has long been a difficult goal to achieve. During the operation of such printers, the fluid supplied to the manifold must be maintained at a pressure in excess of atmospheric pressure in order to produce the necessary flow of fluid through the orifices to form the jet stream filaments. If a multiple print head system is utilized, such as the system taught by Taylor et al, U.S. Pat. No. Re. 28,219, there is a need to maintain substantially equal pressures in each print head during operation so that drop generation remains correlated to predetermined operating parameters.
At startup, if the flow of ink to the orifices is commenced by simply opening a supply line to an empty manifold, it will require a finite time period before normal operating pressure is reached. During that time, ink will initially weep from each orifice and form pendulous masses of material along the bottom of the orifice plate. As pressure increases inside the manifold, uncontrolled jets of ink will finally be expelled from the orifices, stabilizing only after the excess ink along the underside of the orifice plate is carried away by entrainment with the jets.
At shutdown, the opposite problem occurs. If the ink supply to the manifold is merely turned off, pressure will gradually decrease collapsing the flowing jets of ink and concluding with the dribbling of ink from the orifices. Obviously, ink spattering will occur which can cause electrical shorts, burnouts, and ink residue buildup on parts.
Various efforts have been made in the prior art to solve the problems of startup and shutdown. Examples are Culp, U.S. Pat. No. 3,618,858, which teaches the use of open-sided charge electrodes which are moved out of the way during startup and shutdown; Stoneburner, U.S. Pat. No. 3,891,121, which teaches pumping air into the manifold at a high pressure, following the air with a flushing fluid, and then following the flushing fluid with the ink at startup and switching from ink to a flushing fluid and then evacuating the manifold at shutdown; Perry et al, U.S. Pat. No. 4,042,937, which teaches the build up of an initial large pressure behind an inlet valve to the ink manifold which is then opened for startup and the simultaneous closing of the inlet valve and opening of an outlet valve connected to a source of vacuum during shutdown; and Yu, U.S. Pat. No. 4,240,082, which teaches the delayed opening of an outlet valve to a source of vacuum during shutdown.
However, none of the prior art procedures have proved entirely satisfactory entailing the use of additional hardware, prolonged startup and shutdown times, and/or the need for purging procedures to remove air and contaminants from the ink supply manifold. Accordingly, there remains a need in the art for a fluid system in an ink jet printer which provides for both a rapid and clean startup and shutdown while maintaining operating pressures in the print head or heads within predetermined limits.